1. Field of the Invention
The present invention relates to the reception of DIDON data and in particular to a DIDON decoder which comprises a demodulator followed by a demultiplexer. The demodulator receives the television signal, for example from the peritelevision socket connector of a standard television receiver and has the function of delivering at its outputs a binary train of data, a binary clock signal in phase with the bits of the binary train, a validation signal and a line-synchronization signal. The demultiplexer which follows the demodulator has the functions of "chopping" or separating the data of the binary train into octets by detecting the particular configuration of the "synchro-octet" octet, of recognizing the identifier of the source of information for which it is programmed, and of delivering at its outputs data octets derived from said source and excluding data derived from other sources from which the data are transmitted by time-multiplexing in the television signal in the same manner as the data of the source considered.
The invention is more specifically concerned with a particular structure of the DIDON digital demodulator.
2. Description of the Prior Art
A few preliminary remarks recalling the basic principles of the DIDON system will prove useful in order to gain a clear understanding of the description given hereinafter.
The transmission of digital information by radio-wave broadcasting and if necessary with time-multiplexing between a number of sources in accordance with the DIDON procedure will be considered in the case in which the transmission carrier is the television signal. In the DIDON procedure, the digital information is chopped so as to form packets. One packet is transmitted per horizontal scanning line and one complete packet emanates from one source alone. The information contained in a packet is regrouped so as to form octets. The packet is characterized by its header which comprises in sequence: two octets which are characteristic of the binary frequency (synchronization burst), one octet for initialization of chopping into octets ("synchro-octet" octet), three octets for identification of the packet and two octets for validation of the packet (one continuity-index octet and one useful-data format octet). The useful-data octets follow the packet header.
The demodulators employed in the DIDON procedure possess knowledge of the binary nature of the signal to be detected and are provided with the spectral information of the synchronization burst. The design function of these demodulators is to identify the data-transmission lines, to restitute the binary levels and to regenerate the clock signal in phase with the data
The spectrum of the so-called NRZ (nonreturn to zero) signal is continuous, has a first zero at the binary frequency and conveys 80% of the energy of the signal between the continuum and the binary half-frequency. Detection of the binary half-frequency is therefore characteristic of the presence of a line of data. This search for the binary structure is facilitated at the start of a packet by the structure of the first two octets of the header constituted by sixteen "ones" and "zeros" in alternate sequence. It is readily apparent that, in some parts of the image, the signal of the televised program can transport a spectral line at a binary half-frequency. Identification of the data packet is therefore necessarily vitiated by a certain probability of errors which is usually minimized by permitting search and detection only during opening of a window initiated by means of the horizontal synchronization signal and surrounding the presumed position of the binary synchronization burst. The validation signal is produced at the instant of detection.
The transitions of the burst signal on a data-transmission line are employed for regenerating the binary clock signal which is intended to be maintained throughout the time-duration of the data packet.
A variant of these arrangements consists in converting the NRZ signal to a so-called RZ signal (that is, with Return to Zero). The spectrum of a signal of this type has the distinctive property of being constituted by a continuous spectrum in which the initial zero has twice the binary frequency, and on which is superimposed a spectrum of lines having odd-numbered integral multiples of the binary frequency. An oscillating circuit which is resonant at the binary frequency therefore makes it possible both to detect the presence of data and to regenerate the binary clock signal. However, the entire packet must necessarily carry a sufficient number of transitions to ensure that the oscillations are maintained along the entire line.
The binary levels are identified by comparison between the signal transmitted over a data line and a threshold.
The data signal at the output of the television receiver is evaluated by means of its "eye diagram". It is recalled that the eye diagram is the superposition during a single binary period of all the bits transmitted over a sufficiently long time interval. The amplitude and phase distortions sustained by the signal throughout the transmission together with the added noise produce a contraction of the eye diagram both vertically and horizontally. Restitution of the binary levels will therefore be more reliable as the threshold and sampling instant are more accurately centered on the axes of maximum opening of the eye.
Automatic gain control of the television receiver has the effect of considerably modifying the amplitude of data as a function of the mean luminance of the image. Under extreme conditions, the variation in amplitude is 9-dB.
By reason of the vertical contraction of the eye diagram and of the automatic gain control of the television receiver, the threshold is necessarily adaptive.
By reason of the horizontal contraction of the eye diagram, the regenerated clock phase must be as accurate as possible.
Different DIDON demodulators are known at the present time.
The Thomson-Efcis integrated circuit TEA 2585/86 makes use of an oscillating circuit at the frequency FB/2 (where FB is the binary frequency of the DIDON signal; this frequency is an integral multiple of the television line frequency) for the purpose of identifying the line of data. The binary clock signal is delivered by a quartz oscillator at the frequency 2.FB, followed by a logic system for selecting from the four transitions which are present in the same period the particular transition which is nearest the vertical axis of the eye diagram. The circuit detects the mean value of the binary synchronization burst in order to define the threshold with respect to the horizontal axis of the eye diagram. In this demodulator, the threshold has relatively low sensitivity to noise and the clock signal is maintained stable irrespective of the data content. On the other hand, the clock phase is lacking in accuracy.
The integrated circuit SN 96533 produced by Texas Instruments utilizes an oscillating circuit which is resonant at the frequency FB and is excited by a signal with return to zero (RZ). The circuit detects the peak value of the synchronization burst in order to define the threshold. In this demodulator, the clock signal is extracted from the RZ signal itself, with the result that its phase is well-adapted to the eye diagram and follows the fluctuations of the signal. On the other hand, the threshold is sensitive to noise by reason of the peak detection and the clock signal is degenerated if the data signal carries only a small number of transitions.
The demodulator with discrete components produced by CCETT utilizes an open delay-line for identifying by correlations the binary half-frequency synchronization burst. The binary clock signal is delivered by a quartz oscillator at the frequency 8.FB, followed by a scale-of-eight divider in phase with a leading edge of the identified burst. The threshold is defined by detection of the mean value of the burst. In this demodulator, the threshold is relatively insensitive to noise. The clock is stable irrespective of the structure of the data signal and its phase is relatively precise. On the other hand, the clock is not automatically adapted to the data signal since its phase is established once and for all at the beginning of the line.
Among these known demodulators, the type last mentioned, or in other words the demodulator comprising discrete components, is best suited for reception of a signal in which the eye closes or contracts. This demodulator is utilized at the present time as a reference circuit. Monolithic circuits do not have the requisite quality for receiving signals in a disturbed medium. The Thomson circuit offers better noise resistance than the Texas Instruments circuit which, on the other hand, does afford higher resistance to phase distortions than the Thomson circuit.
All these known demodulators call for fine adjustments at works (oscillating circuits, clock phase, phase of validation with respect to line synchronization, some of which involve compromise solutions); they are subject to variations in time.
Furthermore, since they are of the analog type, these demodulators are sensitive to temperature variations.